1. Field of the Invention
The present invention relates to a rubber composition and a heavy duty pneumatic tire comprising the rubber composition. More particularly, the present invention relates to a rubber composition advantageously used for tread rubber of heavy duty pneumatic tires and a heavy duty pneumatic tire comprising the rubber composition in tread rubber.
2. Description of the Related Arts
To improve abrasion resistance and the life of a heavy duty pneumatic tire used for trucks and buses, it is important that rigidity of the tread is enhanced, a decrease in the life due to damages by outside causes is prevented and elevation of temperature of tread rubber due to repeated deformation during driving is suppressed.
Heretofore, to increase the life of a tire, an amount of, for example, carbon black, a resin, a vulcanizing agent such as sulfur, or an auxiliary vulcanizing agent used in the rubber composition is increased. When the amount of carbon black is increased, however, the low heat generating property and heat resistance inevitably deteriorate although abrasion resistance is improved. When the amount of either a vulcanizing agent such as sulfur or an auxiliary vulcanizing agent is increased, heat resistance decreases while the low heat generating property and abrasion resistance are not affected. When the amount of a resin is increased, abrasion resistance and the low heat generating property deteriorate although heat resistance is improved. It is also known that, when natural rubber exhibiting an excellent maximum elongation at high temperatures is blended with SBR exhibiting a high hardness at small deformations, a problem arises with respect to the low heat generating property although abrasion resistance and heat resistance are improved.
For tread rubber of heavy duty pneumatic tires, in general, an isoprene rubber such as natural rubber is used as the rubber component and a combination of carbon black and silica is used as the reinforcing filler so that an excellent balance between abrasion resistance and the low heat generating property is achieved.
A rubber composition comprising an isoprene rubber as the rubber component, however, has a problem in that the modulus tends to decrease due to reversion under over-cure and the low heat generating property tends to deteriorate. In particular, in tires of large sizes such as tires having a total gauge at the hump position in the tread portion of 50 mm or greater, problems arise in that the low heat generating property deteriorates and abrasion resistance decreases.
It is frequently conducted that a loss lowering agent (an agent for providing the property of less heat generation) such as 5-nitroso-8-hydroxyquinoline as a typical example is added to the above composition in order that the low heat generating property of a heavy duty tire is improved. The effect of conventional loss lowering agents such as that described above, however, varies depending on the degree of vulcanization and a problem arises in that the effect of improving the low heat generating property is not sufficiently exhibited for tires , in particular, of large sizes since the modulus decreases due to over-cure which tends to take place in production of such tires.
The present invention has an object of providing a rubber composition which exhibits improved abrasion resistance, tear resistance and low heat generating property and is advantageously used for tread rubber of heavy duty pneumatic tires, and it is another object to provide a heavy duty pneumatic tire using the rubber composition for tread rubber.
As a result of intensive studies by the present inventor, it was found that the objects can be achieved by a rubber composition which comprises a rubber component comprising a modified styrene-butadiene copolymer rubber having a tin atom introduced into the molecular chain, which is obtained by a solution polymerization and natural rubber or an synthetic isoprene rubber as the essential components at a specific compounding ratio and a hydrazide compound.
The present invention has been completed based on this knowledge.
The present invention provides a rubber composition which comprises (A) a rubber component comprising 15 to 55% by weight of (i) a modified styrene-butadiene copolymer rubber having a tin atom introduced into the molecular chain, which is obtained by a solution polymerization, 45 to 85% by weight of (ii) at least one rubber selected from natural rubber and synthetic isoprene rubbers and (B) a hydrazide compound.
The present invention also provides a heavy duty pneumatic tire which comprises a tread rubber comprising a rubber composition described above.